1. Field of the Invention
This invention relates to a highly-airtightened porous paper used principally as an industrial material such as a battery separator, a separator used in an electrolytic capacitor or various types of filters and, more particularly, to novel paper that has minute pores, denseness, and a high degree of airtightness and is made from cellulose which has superior heat and chemical resistance and is a reproducible natural resource.
Further, the present invention relates to a non-aqueous battery. More particularly, this invention is intended to improve various characteristics of the non-aqueous battery, such as heat resistance, ion transmissivity, liquid-holding characteristics, and an internal short circuit, to a greater extent by use of a separator which has minute pores, denseness, and a high degree of airtightness and is made from cellulose which electronically separates the activity of a positively active substance from a negatively active substance.
2. Description of the Background Art
Paper is one of the most familiar articles and is used for packaging foods or beverages or decorating a house, to say nothing of being used as newspapers or book papers. Further, the paper is used as an industrial material in many applications and is one of modern sciences subjects of study. In general, paper is manufactured by dispersing into water cellulose which is prepared by cooking vegetable fibers with chemicals, removing the wet paper web from a papermaking slurry with a screen, and drying this scooped wet paper web.
The cellulose fibers of the paper are bonded together primarily by means of the hydrogen bonding of the cellulose. Specifically, when water evaporates from the wet paper web during the drying process, adjacent cellulose fibers are forcefully attracted together by the strong surface tension of the water. When the distances between the fibers are reduced, Van der Waal's force acts on the fibers to thereby attract the fibers together to a much greater extent. Finally, the fibers are brought into close contact with each other by hydrogen bonding. The degree of hydrogen bonding and the diameter of the fiber determine the extent of the air gap between the fibers, i.e., the degree of airtightness of the paper.
Cellulose which forms paper is a natural macromolecule and is able to resist heat in the vicinity of 230.degree. C. and it has a high resistance to chemicals such as acid, alkaline, or organic solvents. For these reasons, paper is widely used as an inexpensive industrial material, such as a separator used in a battery or an electrolytic capacitor, or as various types of filters.
A porous film is also used as an industrial material in the same applications as the paper. This porous film is as thin as 10 to 40 .mu.m, and minute pores of about 0.1 .mu.m in size are uniformly formed on the entire film surface. Accordingly, the film is used as a filter. Further, in spite of the high degree of electrical insulating characteristics of the film itself, the electrical resistance of the film when it is impregnated with an electrolyte is low. For this reason, the porous film is used as an industrial material, e.g. as separators used in various types of batteries.
Thermoplastic resin included in petroleum resin or a cellulose derivative such as cellulose acetate is used as material for the porous film. A porous film is manufactured from thermoplastic resin by heating and fusing thermoplastic resin in the form of a film, and dissolving a previously drawn or mixed inorganic substance into the acid. When a porous film is manufactured from a cellulose derivative, a film is formed by dissolving the cellulose derivative into a solvent such as acetic acid or acetone, and drawing the thus-formed film.
In addition, non-aqueous batteries, such as lithium batteries or lithium ion secondary batteries, are compact and lightweight and have a large energy density by weight. It is for these reasons that in recent years, the non-aqueous battery has sprung into wide use as a power source of portable electronic equipment, such as cellular phones, notebook computers, or self-contained video cameras. The volume of production of the non-aqueous battery significantly increases year by year. The non-aqueous battery uses as an electrolyte a non-aqueous solvent; e.g., an aprotic organic solvent such as propylene carbonate, methyl ethyl carbonate, ethylene carbonate, methyl propionic acid, .gamma.-butyrolactone, or diethoxyethane. A substance, such as LiBF.sub.4, LiPF.sub.6, or CH.sub.3 SO.sub.3 Li, is dissolved as an electrolyte into the foregoing solvent. A lithium-contained oxide substance such as LiCoO.sub.2 or LiNiO.sub.2 is used as the positively active substance, and a carbon material such as graphite is used as the negatively active substance.
Elements which determine the performance of the non-aqueous battery include the prevention of an internal short circuit as a result of the positively active substance coming into contact with the negatively active substance for the purpose of reducing the proportion of short-circuit failures; retaining the sufficient and required amount of electrolyte for an electromotive reaction; and ensuring a superior transmissivity of a charge carrier required for the reaction of a battery, i.e., a reduction in the impedance, or particularly equivalent series resistance (hereinafter referred to as ESR), in order to reduce the internal resistance of the battery without impeding the transmission of ions. The proportion of short-circuit failures to ESR's is greatly dependent on the separator.
Short-circuit failures have two types of proportions: the proportion of short-circuit failures which occurred at the time of assembly of the battery and the proportion of short-circuit failures which occurred at the time of the use of the battery in the market. In either case, a short circuit arises in the weak portion of the separator. For example, if a pin hole is formed in the separator, a short circuit occurs in the pin hole. To reduce the proportion of short-circuit failures, there is a demand for a separator which is formed as uniformly as possible and has a high density without a pore such as a pin hole. In other words, there is a demand for a separator having a high degree of airtightness.
Contrary to the improvement of the proportion of short-circuit failures, in order to reduce the ESR, a porous separator, i.e. a separator having a low degree of airtightness is demanded to ensure pores which permit the passage of ions. This is due to ionic conduction, which is effected in the non-aqueous battery, where electric charges move as a result of the transmission of charged ions in the non-aqueous battery. As described above, there are two contradictory demands for the separator, i.e., a separator having a high degree of denseness or a high degree of airtightness in order to reduce the proportion of short-circuit failures, and a porous separator having a low degree of airtightness in order to improve the ESR of the battery.
A polyolefine-based porous film or unwoven fabric is used as a separator which has a high degree of airtightness and pores and is to be used in a non-aqueous battery. More specifically, porous polypropylene or polyethylene film is commercially available. A polyolefine-based unwoven fabric is principally used in coin-type batteries, and a polyolefine-based porous film is principally used in cylindrical batteries.
The polyolefine-based porous film is as thin as 10 to 40 .mu.m, and minute pores are uniformly formed on the entire film surface, so that an air resistance of thousands of seconds/100 cc to tens of thousands of seconds/100 cc are obtained. In spite of a high degree of electrical insulating characteristics of the film itself, the electrical resistance of the film when it is impregnated with an electrolyte is low. For this reason, the porous film is used as a separator.
If the polyolefine-based porous film is heated to an abnormally high temperature, the film is fused at an internal temperature of about 120 to 170.degree. C., thereby resulting in a shut down effect in which the minute pores will close to thereby prevent the flow of an electrical current. This shut down effect acts as a safety mechanism.
However, since the existing separator is formed from polyolefine-based substances, the separator has a fusing point (polyethylene does not have a resistance to temperature, at most, it is 120.degree. C., whereas polypropylene has a resistance to a temperature of 160.degree. C.) and lacks dimensional stability. Accordingly, if the internal temperature of the existing separator becomes high, shrinkage deficiencies arise in the porous film, and an internal short circuit can occur in the shrinkage deficiency. An electrical current concentrates around the short-circuited area before the battery becomes completely shut down, thereby resulting in an upsurge in the internal temperature. The polyolefine-based porous film holds the risk of becoming fused and abnormally heated. Further, if the temperature of the film has reached a temperature of 130.degree. C. or more, the porous film may become fused and effluent. An internal short circuit occurs across both polarities, thereby building a fire. Therefore, in order to greatly improve the degree of safety, there is a demand for a separator of a non-aqueous battery holding a higher degree of heat resistance and dimensional stability.
Further, there are desired increases in the volume of the non-aqueous battery and a reduction in the size and weight of the same, and hence the reduction in the thickness of the separator is even more desirable. Originally, pin holes of the order of micrometers, more specifically, a plurality of pin holes like oval cracks measuring 0.5 .mu.m in major axis and 0.05 .mu.m in minor axis, are distributed over the existing polyolefine-based porous film. The pin holes of the order of micrometers permit the passage of minute particles of active substances of both polarities, which in turn may reduce the volume or lifetime of the battery or cause an internal short circuit. If the thickness of the separator is reduced, the rate of occurrence of pin holes increases, and the size of the resultant pin holes also becomes larger, thereby rendering the separator more apt to cause a short circuit. Therefore, it becomes impossible to respond to the demand for a thinner separator. Since the polyolefine-based porous film is not lyophilic and is not impregnated with an electrolyte well, the amount electrolytes held in the battery reduces, thereby shortening the lifetime of the battery.
To improve the safety of this product, it is desirable that the non-aqueous battery holds a higher degree of heat resistance. More specifically, in the case of a lithium-ion battery, there is a demand for a separator that retains its shape at a temperature of 190.degree. C. or more which is the ignition point of lithium. At present, there are no porous films which have such heat resistance. Polyethylene or polypropylene is an expensive material and requires a complex manufacturing process, which makes it difficult to reduce its cost. These days, new materials are sought after, since polyethylene and polpropylene are pertroleum resources which can negatively effect our environment.
Cellulose is a reproducible and inexpensive material which has both heat and chemical resistance. The cellulose is a material which has resistance to heat up to a temperature of 230.degree. C. As can be seen from the fact that a chemical agent which dissolves cellulose is still sought, the cellulose is stable with respect to chemical contact. If a separator which has minute pores, denseness, and a high degree of airtightness can be formed from cellulose, the proportion of short-circuit failures and ESR of the battery are reduced, thereby improving various characteristics of the separator, such as heat resistance, ion transmissivity, liquid-holding characteristics, and prevention of an internal short circuit to a much greater extent. However, the separator manufactured by an existing papermaking method cannot simultaneously satisfy the demand for pores and the demand for increased airtightness. If an attempt is made to increase the degree of airtightness to 1000 sec/100 cc in order to prevent an internal short circuit and the proportion of short-circuit failures while paper is formed to a thickness of 100 .mu.m or less which makes it possible to use the paper as a separator for non-aqueous battery, the cellulose pulp must be beaten to a density of about 0.75 g/cm.sup.3. As a result, the separator becomes a film, and pores which permit the passage of ions are lost, thereby deteriorating the ESR of the film.
In a case where a separator for a non-aqueous battery is manufactured from cellulose, controlling the airtightness of the separator is an important factor in determining the performance of the separator. To improve the proportion of short-circuit failures and the ESR of the battery, it is desirable to have a separator with minute pores and a high degree of airtightness, more specifically, an airtightness of 1000 sec/100 cc or more. This is because a separator having an airtightness of hundreds sec/100 cc has pinholes even if it has denseness as a whole.
In a case where paper is used as an industrial material such as a separator for a battery, the control of airtightness of the paper is important. The separator used in a battery for separating a positively active substance from a negatively active substance within the battery is strongly desired to have a certain denseness in order to separate the active substances from each other. Particularly, the standard requirement for a separator which is used in a lithium-ion battery, is that the separator must have an airtightness of 1000 sec/100 cc or more in order to ensure the denseness of the separator.
The airtightness of paper made from cellulose is controlled by the following two existing methods. One method is manufacturing a separator with a higher density, by further beating cellulose fibers, and the other method is controlling the airtightness of the paper by increasing the thickness of the separator.
In reference to controlling the airtightness of the paper by adjusting the extent of beating the cellulose, if a separator with a low density is made from more softly beaten cellulose fibers, the separator will have a lower degree of airtightness. In contrast, if a separator with a high density is made from sufficiently-beaten cellulose fibers, the separator will have a higher degree of airtightness. If a separator which has a density of 0.3 g/cm.sup.3 and a thickness of about 50 .mu.m is made from virgin pulp which has a value of 770 ml as CSF specified by JIS P8121 (Canadian Standard Freeness which will be hereinafter referred to as JIS-CSF), i.e., virgin pulp whose cellulose fibers are not substantially beaten, the airtightness of the paper can be controlled to about 1 sec/100 cc. If a separator is manufactured by beating the cellulose fibers to a JIS-CSF value of about 400 ml, the density of the separator can be increased from 0.3 g/cm.sup.3 to 0.55 g/cm.sup.3 and the airtightness of the separator can be controlled to hundreds of sec/100 cc, given that the thickness of the separators are the same.
Conclusively, the airtightness of the paper can be controlled from thousands of sec/100 cc to tens of thousands of sec/100 cc or more if the cellulose fibers are beaten to a greater extent. However, if highly beaten raw materials are used for manufacturing the separator, pores which can penetrate the separator, disappear. For this reason, it is impossible for the existing separator to achieve an airtightness of greater than 1000 sec/100 cc. If cellulose fibers are beaten to a JIS-CSF value of about 200 ml or more and a separator is manufactured from these cellulose fibers, the air gaps between fibers disappear, which in turn prevents pores from being formed in the separator. Then the airtightness becomes infinite, which makes it impossible to actually measure the airtightness. This problem is inevitable, so long as the separator is manufactured from cellulose possessing autohesion (the capacity to bond together). The disappearance of pores results in the disappearance of passages for ions, thereby, considerably deteriorating the ESR of the battery.
In general, as the diameter of the fibers become smaller, a greater forces act on the voids between the fibers of the wet paper web. This phenomenon is known as the Campbell effect. According to the calculation of the Campbell effect, the attracting force acting on fibers having a diameter of 30 .mu.m is 6.1 kg/cm.sup.2, whereas the attracting force acting on fibers having a diameter of 2 .mu.m is 38 kg/cm.sup.2. If the diameter of the fiber becomes 0.2 .mu.m, the attracting force acting on the fibers becomes 174 kg/cm.sup.2. In comparison with the original size of vegetable fibers, the size of highly beaten vegetable fibers becomes smaller, and the attracting force acting between the fibers becomes larger, and hence the distance between the fibers also becomes reduced. When the wet sheet enters the drier process, the remaining water evaporates. Since the surface tension of water is great, the adjacent fibers are strongly attracted together. When the distance between the fibers decreases, Van der Waal's force acts on the fibers, thereby further attracting the fibers together. Finally, the fibers are brought into close contact with each other by hydrogen bonding, so that the voids between the fibers are reduced. If the fibers are beaten to a JIS-CSF value of 200 ml or less, the voids between the fibers of a resultant separator disappear. Consequently, it becomes impossible to measure the density of the separator. Pores which permit the passage of ions disappear. In contrast, when the fibers are more softly beaten and the profile of the fibers can be retained, there still exists a plurality of voids in the fibers as a whole even if fibers make hydrogen bonds with each other at points where they come into contact.
Even if a JIS-CSF value is minutely adjusted before the value of fibers reaches a JIS-CSF value of 200 ml, the airtightness of the separator cannot be controlled to a value greater than 1000 sec/100 cc. As described above, when the diameter of fibers decreases, the force acting between the fibers sharply increases. Further, when the cellulose fibers are beaten, cellulose fibers are not cleaved into two's or three's stepwise. Fibrils having a diameter of about 0.4 .mu.m grow stepwise from the outer surface of the fiber in much the way that whiskers do. More specifically, the extent to which the cellulose fibers are beaten is the state of occurrence of fibrils having a diameter of 0.4 .mu.m. The progress in the extent of beating of the cellulose fibers represents an increase in the proportion of fibrils. In contrast, cellulose fibers from which paper is made, e.g., fibers of conifers, have an oval shape and measure 40 .mu.m in major axis and 10 .mu.m in minor axis. Fibers of Manila hemp pulp have a substantially circular shape and a diameter of about 20 .mu.m. In the case of the Manila hemp pulp, the extent of beating can be grasped as a variation in the rate of fibers having a diameter of 20 .mu.m to fibrils having a diameter of 0.4 .mu.m. Therefore, the airtightness of the paper cannot be controlled by minutely adjusting the JIS-CSF value before the value reaches a JIS-CSF of 200 ml. Even if an attempt is made to control the airtightness of the paper, it is thought that variations ranging from plus or minus thousands to tens of thousands sec/100 cc will arise.
For this reason, it is possible to manufacture a separator having an airtightness of hundreds of sec/100 cc by controlling the extent of the beating, but it is impossible to manufacture a separator by controlling the airtightness to one thousand to tens of thousands sec/100 cc while retaining the pores which enable the passage of ions. More specifically, it is impossible to manufacture, from cellulose, a separator which has pores and a high degree of airtightness.
Another existing method of increasing airtightness is to increase the thickness of a separator. Theoretically, as the distance through which air travels becomes longer, airtightness becomes higher. Accordingly, if the thickness of the separator is increased, it is possible to manufacture a separator which has a high degree of airtightness. However, in a case where paper is used as an industrial material such as a separator used in a battery, it is most desirable that the paper is as thin as possible. Paper having the thickness ranging from 15 to 100 .mu.m is principally used as the separator. For example, a porous film principally used as a separator of a lithium-ion battery generally has a thickness of 25 .mu.m, and a separator principally used in an electrolytic capacitor has a thickness of 15 to 90 .mu.m. In effect, paper having a thickness smaller than those of the foregoing separators, cannot be used as a separator. Particularly, at the present time, there is a demand for a battery which has a larger volume and is compact and lightweight, and therefore it is expected that the thickness of paper will be reduced further. Accordingly, it is impossible to control the airtightness of paper to a value of 1000 sec/100 cc or more within the thickness range of 100 .mu.m or less, which is required for a separator when it is used as an industrial material, by adjusting the thickness of the paper or by adjusting the extent of the beating of cellulose fibers and the thickness of the paper in combination.
For these reasons, the foregoing porous film is currently used as an industrial material, such as a battery separator or as various types of filters, which are porous and require a high degree of airtightness. A film which has an airtightness which ranges from thousands of sec/100 cc to tens of thousands of sec/100 cc can be used as a porous film.
Thermoplastic resin included in petroleum resin or a cellulose derivative such as cellulose acetate is used as the material for porous film. Polyethylene (PE) or polypropylene (PP) is principally used as a thermoplastic resin for the petroleum resin. This type of resin possesses superior resistance to chemicals but a low resistance to heat. Specifically, polyethylene has a resistance to temperature of, at most 120.degree. C. and polypropylene has a resistance to temperature of 160.degree. C. In contrast, although cellulose acetate which is one of the cellulose derivatives has resistance to temperature in the vicinity of 230.degree. C., it lacks a resistance to chemicals because the cellulose acetate is dissolved in an acetic acid or acetone. For these reasons, a porous film made from a cellulose derivative cannot be used as a battery separator. As described above, if a porous film has a superior resistance to chemicals, it may have a low resistance to heat. In contrast, if a porous film has a superior heat resistance, it may have a low resistance to chemicals. There are no porous films possessing resistance to both heat and chemicals. Further, thermoplastic resin which is the material for porous film is expensive and requires a complicated manufacturing process. Therefore, it is difficult to reduce the manufacturing cost of the film.
In contrast, it is expected that a separator, or the like, which has a higher degree of heat resistance will be developed, in order to improve the safety of industrial products such as batteries. For instance, in the field of the lithium-ion battery, there exists a demand for a separator which retains its shape at a temperature of 190.degree. C. or more at which lithium catches fire. Presently, there is no porous film with a heat resistance that can respond to this demand. Both polyethylene and polypropylene are petroleum-based resources, and a new material is sought in terms of environmental consideration.
Table 5 shows the results of a comparison of the properties between paper manufactured by a conventional method and porous film manufactured from a thermoplastic resin of the petroleum-based resin.
As shown in Table 5, cellulose used as the material of the film has resistance to heat up to a temperature of 230.degree. C. As can be seen from the fact that a chemical agent which dissolves cellulose is still sought, it is said that the cellulose is stable with respect to chemicals and has a resistance to both heat and chemicals. In contrast, the porous film inherently lacks resistance to heat and chemicals. Provided that high-density paper which is manufactured from highly beaten material and has an infinite and unmeasurable airtightness can be formed to be porous, then, paper which has a high degree of airtightness and a low degree of density and is presently impossible to manufacture, can be produced. More specifically, if paper with minute pores which permit the passage of air, can be manufactured even from highly beaten material, paper having a high degree of airtightness and a low degree of density can be produced. This paper would have a high degree of airtightness and a low degree of density, and would enable a high level of control of the airtightness. As practiced in the prior art, if the airtightness of paper is increased, the density of the paper is also increased, thereby deteriorating the electrical characteristics of the paper. In contrast, if the density of the paper is reduced to improve the electrical characteristics of the paper, the airtightness of the paper reduces, thereby resulting in an insufficient denseness of the paper. Therefore, the paper having a high degree of airtightness and a low degree of density will make it possible to eliminate the deficiency of the paper, i.e., the difficulty in satisfying improvements in the airtightness and the electrical characteristics of the paper at one time. Further, such paper can be used in fields where a porous film cannot be used, owing to its insufficient heat resistance or can contribute to improvements in the safety of products in which the paper is already used. Simultaneously, the paper is desirable because it enables the conversion of petroleum resources to reproducible natural resources.
In view of the foregoing problems in the background art, the object of the present invention is to provide highly-airtightened porous paper which was produced from a reproducible natural resource, i.e., cellulose having superior resistance to heat and chemicals, and has minute pores and a low degree of density. More specifically, the object of the present invention is to provide highly-airtightened porous paper which has a thickness of 100 .mu.m or less and an airtightness of 1000 sec/100 cc or more.
To manufacture a porous separator from cellulose and in order to improve the ESR of the separator, it is necessary to produce a thin separator which has a low degree of density in contrast with the case where the proportion of short-circuit failures is improved. However, if the thickness or density of the separator is reduced, the airtightness of the separator will be inevitably reduced. Further, if the thickness of the separator is increased to improve the airtightness of the separator, the ESR of the separator will deteriorate like a linear expression. In contrast, if the density of the separator is increased, the ESR will deteriorate like a second-order equation.
As described, in the prior art, it is impossible to manufacture a porous and highly-airtightened separator from cellulose, which has pores to permit the passage of ions. Therefore, it has been impossible to realize a high level of improvement in both the proportion of short-circuit deficiencies and in the ESR of the separator.
Accordingly, provided that high-density paper which is manufactured from highly beaten material and has infinite and unmeasurable airtightness can be formed to be porous, a separator which has a high degree of airtightness and a low degree of density and is presently impossible to manufacture can be produced. More specifically, if a separator with minute pores which permits the passage of air can be manufactured even from a highly beaten material, a separator having a high degree of airtightness and pores for the passage of ions can be produced from cellulose. This separator which will have a high degree of airtightness and a low degree of density will enable a high level of control in airtightness. As practiced in the prior art, if the airtightness of the separator is increased, the density of the separator is also increased, thereby deteriorating the ESR of the separator. In contrast, if the density of the separator is reduced to improve the ESR of the separator, the airtightness of the separator reduces, thereby resulting in an sufficient denseness of the separator. Therefore, a separator having a high degree of airtightness and a low degree of density will make it possible to eliminate the deficiency of the separator, i.e., the difficulty in realizing a high level of improvement in the proportion of short-circuit deficiencies and the ESR of the separator at one time. Further, cellulose is a reproducible natural resource and does not present a problem associated with industrial waste. Therefore, such a separator is desirable because it enables the conversion of petroleum resources to reproducible natural resources.
Accordingly, in view of the foregoing problems in the background art, the object of the present invention, is to provide a novel, highly-airtightened porous separator which will be produced from a reproducible natural resource, i.e., cellulose having superior resistance to heat and chemicals, and has minute pores and a low degree of density. More specifically, the object of the present invention, is to provide a non-aqueous battery which will be improved to a higher level with regard to various characteristics, e.g., heat resistance, electrical characteristics such as ion transmissivity or liquid-holding characteristics, or prevention of an internal short circuit, through use of a highly-airtightened separator.